The present invention concerns a method of producing an oil-in-water microemulsion from a microemulsion concentrate, and a method of employing such a microemulsion to recover hydrocarbons from underground formations.
A microemulsion is a thermodynamically stable, clear or translucent micellar solution comprised of oil, water, and surfactant. The microemulsion may also contain one or more electrolytes, cosurfactants, and when used for enhanced oil recovery, water-soluble polymers and sacrificial agents. The droplet diameters of the dispersed phase range from roughly 140 to 10 nanometers.
Microemulsions may be classified as oil-in-water microemulsions, water-in-oil microemulsions, and middle-phase microemulsions. An oil-in-water microemulsion is a microemulsion in which the external or continuous phase is water and the dispersed phase is oil. A surfactant-water-mixture which forms an oil-in-water microemulsion can equilibrate as a single phase or as two phases such as a lower phase oil-in-water microemulsion plus an equilibrium upper phase of oil, depending on the overall composition of the mixture. Such mixtures are defined by the those skilled in the art of surfactant flooding as existing in a Type II(-) phase environment.
A water-in-oil microemulsion is a microemulsion in which the external or continuous phase is oil and the dispersed phase is water. A surfactant-oil-water mixture which forms a water-in-oil microemulsion can equilibrate as a single phase or as two phases of an upper phase water-in-oil microemulsion plus an equilibrium lower phase of water depending on the overall composition of the mixture. Such mixtures are defined by the those skilled in the art of surfactant flooding as existing in a Type II(+) phase environment.
A middle-phase microemulsion is a microemulsion in which there is apparently no identifiable external or continuous phase. The structure of the middle-phase microemulsion is still unresolved. A surfactant-oil-water mixture which forms a middle phase microemulsion can equilibrate as a single phase, as two phases of a middle-phase microemulsion plus an equilibrium phase of oil or water, or as three phases. The three phases would be a middle-phase microemulsion plus an equilibrium water phase and an equilibrium oil phase. The end result of the equilibrated microemulsion depends on the overall composition of the mixture. Such mixtures are defined by those skilled in the art of surfactant flooding as existing in Type III phase environments or regimes.
A macroemulsion is a thermodynamically unstable, opaque dispersion of two or more usually insoluble liquids, one in the other. It is characterized by its propensity to separate into two or more original liquid phases upon standing. The droplet diameters of the dispersed phase range from roughly 200 nanometers to visually resolvable, discreet aggregates. Because of the propensity of a macroemulsion to separate into its components, macroemulions are not desirable surfactant systems for enhanced oil recovery.
Microemulsions may be used for different chemical purposes. One well known use for oil-in-water microemulsions is to conduct surfactant floods to recover oil from underground formations. It is also believed that oil-in-water microemulsions generally perform better for hydrocarbon recovery when they are formulated with high equivalent alkane carbon number (EACN) oils. Such high EACN oils typically have an EACN 50% to 300% higher than the EACN of the crude oil for which the microemulsion is optimized.
The literature teaches the advantages of using low oil content microemulsions for enhanced oil recovery which are formulated with high EACN oils. U.S. Pat. No. 4,318,816 demonstrates that the stability of a single component surfactant such as an alkylarylpolyalkoxyalkylene sulfonate is enhanced when a high EACN oil is added to an aqueous surfactant to form an oil-in-water microemulsion. The high EACN oil raises the phase inversion temperature of the ethoxylated surfactant. U.S. Pat. No. 4,271,907 teaches that water soluble polymers are compatibly incorporated into an optimized microemulsion only when the microemulsion is formulated with a high EACN oil.
However, oil-in-water microemulsions are difficult to economically produce in the field on a field-wide scale. This difficulty is enhanced when a high EACN oil is used to create the oil-in-water microemulsion.
Since the high equivalent alkane carbon number (EACN) oil is 50% to 300% higher than the crude oil for which the surfactant is optimized, the high EACN oil to brine interfacial tension is high. Because the interfacial tension is high, the high EACN oil is not rapidly microemulsified when added to the surfactant and brine mixture. Instead of forming a microemulsion, a macroemulsion is formed which is useless to an enhanced oil recovery process.
This macroemulsion will eventually be converted to the desired oil-in-water microemulsion if stirred at low shear and ambient temperature for a long period of time, such as several days to several weeks. However, this method is impractical in the field for obvious reasons. It requires a large investment in mixing and storage tanks. The number of tanks needed to hold several days or weeks worth of microemulsion would be prohibitatively large.
One known method of creating a water-in-oil microemulsion with alkoxylated surfactants is to heat and then cool the mixture of surfactant, high EACN oil and brine. When heated, a water-in-oil microemulsion is formed because the alkoxylated surfactants become more hydrophobic as the temperature is increased. Upon cooling, the system is inverted to form a stable, translucent, oil-in-water microemulsion. This method is not practical in the field because large volumes of fluid must be heated to 20.degree. to 50.degree. C. above reservoir temperature.
A second method of producing stable oil-in-water microemulsions is to subject the mixture of surfactant, high EACN oil and brine to a very high shear. For example, a microemulsion having particle sizes less than 100 nm can be formed after multiple passes through a shear device having a presssure drop of about 14,000 psi. Since practical pressure drops obtainable in the field approach only about 2,000 psi, extensive shearing and a large investment in on-site blending and storage facilities would be required to yield injectable oil-in-water microemulsions.
A homogeneous, single-phase oil-in-water microemulsion can also be formed by adding to the water or brine a homogeneous blend of the high EACN oil, and either a "100% active" surfactant or an oil soluble surfactant dissolved in a hydrocarbon. The term "100% active" surfactant refers to a surfactant which contains very little or no water, such as a polyalkoxylated alcohol or an alkylaryl sulfonic acid. These surfactants may contain small amounts of feedstocks and salts, so the surfactant concentration may not actually be 100%. Since the high EACN oil is completely miscible with the 100% active surfactant or the oil soluble surfactant dissolved in hydrocarbon, the high EACN oil is already microemulsified when it is added to the water or brine, and thus the microemulsion forms almost spontaneously.
Although this method is attractive, it can not always be used because many of the surfactants needed for enhanced oil recovery can only be supplied as aqueous concentrates, and not in a 100% active surfactant form. In addition, if the 100% active surfactant or the oil soluble surfactant dissolved in hydrocarbon is but one component of a multi-surfactant blend, it alone may be unable to microemulusify all of the high EACN oil. Most surfactant systems employed in enhanced oil recovery use multiple surfactants.
U. S. Pat. application Ser. No. 944,894, filed Dec. 22, 1986, now abandoned, discloses another way of producing oil-in-water microemulsions. In this method, salt is added to a surfactant, oil and brine mixture in a sufficient quantity to drive the mixture into a Type III or Type II(+) phase regime. Another solution having a selected lower salinity is mixed with the Type III or Type II(+) phase regime to invert the Type III or Type II(+) regime into a Type II(-) oil-in-water microemulsion.
The need for a method to rapidly and economically produce the high EACN oil-in-water microemulsions in the field, on a field-wide scale, is clear. The present invention is a method which provides a significant reduction in on-site construction and operating costs by eliminating the need for heat, extensive shear, and large numbers of storage and blending tanks.